I think you are trying to damp down the stove too much. Keep your air control open more and I bet you get your 600°F cruising temps. FYI, Jotul rates their stoves for a nominal heat output at approx. 40% air open. Optimal temp for most efficient burn is 400-600°F measured at the stove top surface. You should adjust your primary air to keep the stove in that range. Every installation will require a different primary air setting.